Automated production and manufacturing plants in principle comprise the plant components to be automated (production and manufacturing equipment), the automation system and the connection elements between plant and automation system. Such connection elements include sensors and actuators (field devices) and the line components required to connect them, where line components may be, for example, standard copper wires, thermocouple wires, equalizing conductors, compensation units etc. Passive, non-intelligent sensors such as thermocouples, thermistors, pressure sensors, accelerometers, resistance-type sensors, position sensors etc., and non-intelligent actuators such as valves, relays, motors etc. are often used to detect and control process variables such as temperature, pressure, valve position etc.
In order to connect a sensor or an actuator to an input/output module of the automation system, the terminals of the sensor or actuator must be connected to at least the same number of terminals of the input/output module. The lines can be classified here into excitation lines and measurement lines, where sensors usually have another m=0 excitation lines in addition to n=2 measurement lines, because many sensors need to be supplied by currents, frequencies etc. in order to show a measurable response. In addition to m=2 excitation lines for setting the control variable, an actuator has another n=0 measurement lines for monitoring the actuator response. In addition, k=0 lines for both excitation and measurement may be used with many sensors and actuators. Measurement variables and excitation variables may be current, voltage, frequencies etc. for example. FIG. 1 shows a typical design of an installation of a field device 1 to a conventional input/output module 2 of an automation system, where the field-device terminals 3 are connected to the terminals 4 of an excitation component 5 and to the terminals 6 of a measurement component 7, which are connected to a control unit 8.
Some sensors and actuators can also be operated with fewer lines by measurement and excitation being made via common lines. Dispensing with lines usually results in loss of accuracy. For example, with resistance-type sensors, the lower the number of lines used, the greater the measurement inaccuracies caused by line resistances. FIGS. 2 to 4 show typical connections of resistance-type sensors 9 having four, three and two lines. The resistance-type sensors 9 are each excited by a current; a voltage is then measured from which the resistance can be calculated. The m excitation lines 10 are shown here with dashed lines, while continuous lines are used for the n measurement lines 11 and k combined excitation/measurement lines 12.
During installation of a field device (sensor or actuator) there is always the inherent risk of an incorrect connection, with mistakes occurring more easily, the more lines that such a field device has and the more terminals the input/output module has per channel. In addition, faults such as a broken wire or short circuit can arise in the line components during operation.
Irrespective of whether errors arise as a result of incorrect installation or during operation, they usually result in incorrect or inaccurate measurements, which if not immediately identified, significantly impair the quality of the manufacturing process and products, and may lead to production rejects. Significant costs also arise in these cases to locate and repair the source of the impairment. FIGS. 5 to 7 show examples of incorrect installations of four-wire, three-wire and two-wire resistance-type sensors.